In the 1950s, for example, Chinese leader Mao Zedong exhorted the masses to "allow the past to serve the present." Farmers were mobilized to demolish parts of the wall and use the bricks for building houses, pigpens, and walls.

As capitalism began making inroads in the 1980s, many officials believed that tourism money would save the wall. But today the industry may pose the biggest threat to the wall's survival.

Poorly executed restoration efforts have left sections near the capital, Beijing, looking like a Hollywood set. Entrepreneurs have set up cable cars, souvenir stalls, fast-food restaurants, amusement facilities, villas, and crowded parking lots—all within a stone's throw of the structure.

In Gansu province a portion of the wall was rented out to farmers, who "restored" the wall by covering it over with cement, then installed a gate, so they could charge admission.

A short distance away tourists have pulled grass from rammed-earth walls—among the oldest and most endangered segments. And Christmas lights have been nailed to the 14th-century towers guarding Gansu's Jiayuguan Great Wall gate.

In perhaps the most egregious infraction, highway crews in China's Inner Mongolia region have smashed holes in the wall to make way for new roads.

For his new book, The Great Wall Revisited, Lindesay, a photographer by training, gathered hundreds of old photos of the Great Wall. He then set out to rephotograph 150 of the locations in the earlier images, creating sobering pairs of then-and-now scenes.

In some cases, he found that sections had disappeared altogether.

"Delivering the evidence is very important," Lindesay said. "You can really see the wall disappearing."

Turning a Corner?

In 2002 the New York-based World Monuments Fund put the Great Wall on its list of the World's 100 Most Endangered Sites. Chinese government officials sat up and took notice.

"It was a wake-up call," said Lindesay, who's spent more than 1,200 days on the wall over the past 20 years.

In 2003 Beijing announced its first regulations to protect the Great Wall in the capital area. Then last December the central government announced a new national law to protect the wall.

It is now illegal to remove bricks or stones from the wall, carve names in the bricks, hold raves on the wall, or build a house against the wall. Also important, the law says that "all citizens, legal entities and organizations" are charged with protecting the wall and reporting illegal activity to government agencies.

Dong Yaohui, vice chairman of the Great Wall Society of China, said the new rules are significant.

"Now the government has clearly made the protection of the Great Wall a national effort," he says. "The law states what can and can't be done, and it says who's responsible. And it defines society's responsibility to protect the Great Wall."

Dong, who walked the entire length of the wall in 1984-85, said that the drafting of the law is a sign of the government's growing awareness of the problem.

Lindesay, of International Friends of the Great Wall, agreed. "The Chinese are realizing that a lot has been lost."

Already the new law is showing its teeth, or at least its gums.

On December 3 a construction company became the first to be fined under the new rules. For dismantling large pieces of the wall to make way for in illegal highway, the authorities fined the builders the equivalent of U.S. $6,500.

A Cultural Revolution?

The wall's biggest problem today, Dong said, is the lack of understanding among the Chinese, whom he said don't realize the true significance of the Great Wall.

The June survey team, for instance, found parts of the wall covered in Chinese graffiti and farmers carting bricks away from the wall, just as they've been doing for decades.

And in November three men in Inner Mongolia were detained for taking earth from an ancient 2,200-year-old section of the wall to use as a landfill for a village factory.

"It's just a pile of earth," village head Hao Zengjun told the official Xinhua News Agency.

Dong said, "Outside of Beijing and [neighboring] Hebei province, the Great Wall is in very poor and backward areas. Trying to get the significance of the wall across to a people worried about their survival is not easy."